Electric detonators



1960 M. E. EILERS ET AL 2,927,498

ELECTRIC DETONATORS Filed July 26, 195"! INVENTORS MELVIN E. EILERS LESTER E.5MITH BY JUMLY m fi gg ATTYOQ NEYS This invention relates to electric detonators and par- Melvin E. Eilers, Gillespie, and Lester a. Smith, Grassy;

ticularlyjto electric' detonators with improved water- 7 proofness and a method of making the same. 7

Electric detonators usually consist of an explosive train housed in a metal cup provided with a bridge wire imbedded in a sensitive explosive material within the case. The bridgewire isextended between the ends of two lead wires which are supported by' the bridge plug closing the mouth of the case. Morerecently, the, metal detonator case has been replaced with oneformed from an organic thermoplastic material, and the bridge plug is also formed of an organic thermoplastic material.

Detonators of this type provided with a thermoplastic 1 case and plug are particularly well exemplified'in U.S. Patents 2,415,045 and 2,420,201 granted to FrederickjR. Seavey. While the detonators described in these. patents represent a pronounced advance in the field, there is still a definite need' for thermoplastic encased detonators, having positive waterproof seals that can be provided with case on a commercial scale.

It'fis, therefore, an obect of this invention to provide an ,electric blasting cap provided with an'improved closure that overcomes the disadvantages and diflicultiesof I previously known detonators. Anothenobject'of this in vention is to provide anoveldetonator housed in anor'ganic' thermoplastic material. [A further object of this invention is to provide anew and'n'ovel method of sealing felectric'idetohators. Still anotherobject is to provide a method of sealing electric detonators housed in difiicult ly soluble or practically insoluble thermoplastic material. Other objects will .becomeapparent when the following detailed description is read in connecation with the'accompanyingdrawing in'which:

accordance with this invention;

Figure 2 is a sectional view of another embodiment ,of a bridge plug made in accordance with this invention;

figure 1 is a sectional view of a bridgeplug made in .Figure 3 is a sectional longitudinal view of an' assem bled detonator in position preparatory to sealing; and

Figure 4 is a sectional longitudinal view of an electric detonator representing a'jpreferr'ed embodiment of this invention..-

These objects are accomplished in accordance with this invention generally stated by positioning a ring of metal between the organic thermoplastic plug and the organic thermoplastic case of an electric detonator and heating the metal ring by induced electric currentabove the melting point of the material in contact with it.. In other words, the sealing of an electric detonator in accordance with this invention is accomplished by placing a metal ring about the periphery of a thermoplastic bridge plug intermediate its ends, inserting the bridge plug with the 7 ring about it into the mouth of a detonator case provided with an explosive charge and placing the assembly in a high frequency energy field whereby the metallic ring is heated by an induced current. plastic material surrounding the metallic ring to melt and fusetogether so as to form a positive seal for the deto- This causes the thermo-.

, 2,927,498 Patented Mar. 8

nator. In order to obtain this positive waterproof seal,

the-metallic ring must be in contact with the bridge plug aswell as the case.

In practice, it has been most convenient to provide the bridge plug with a circumferential groove to accommodate the metallic ring. The loop or ring of metal can be'formed of foil or wire wrapped about the bridge plug. Alternatively, the ring can be cast and imbedded in'the periphery of the bridge plug. This ismost readily accomplished by molding the bridge .plug with the metallic ring as an insert between the ends of the plug. Also, the ring can be in the form of a metallic band, foil, or a metallic suspension, such as a metallic paint, on the interior of the case wall or on the side wall of the case. In any event, the metallic ring must be continuous so as to form a non-interrupted loop aboutthe bridge plug..

The heating effect of the current induced into the metal ring is more pronounced when the ring is formed of a high resistance metallic material such as iron or Nichrome (60% nickel, 25% iron, and 15% chromium) and the like. Materials having lowerelectrical resistances can be used with equal success but require high energy output to provide the same heating effect.

The metallic ring between the bridge plug and case is utilized as a secondary coil which generates sufficient heat to cause the thermoplastic material about it tomelt and commingle to effect a seal. The current is induced in the metallic ring by positioning the assembled d etona'tor in the high frequency field of the work coil of an induction heating generator. The detonator is positioned withinthe coil and preferably'near its center so as'to avoid uneven heating of the metallic ring.' The amount of energy imparted to themetal ringcan be controlled within close limits by variation of a number of factors. Thus, the degree of heating can be controlled by variations in the distance between the work coil and the man ring, the number of'turns in the work coil, the cross'sec- V tional area and position ofthe metal ring,"the power-output of the work coil, and the duration of time rdr'app1ication of the energy. For maximum safety, it is 'pi'e ferred to shuntthe ends of the lead wires so as to prevent circuit of the detonator. s p

In order to more fully explain andfurther clarifythe invention, reference is made to the drawing for an illusitrative embodiment thereof. Figures 1 arid-Z show are different types of bridge plugsthat can'be us'e'd in carrythe formation of a secondary current through 'thejtir inig ing outthe present invention. In Figure lithe bridge plug 1 is formed of molded ethyl cellulose and carries a pair of lead wires 2 insulated in sheaths 3 of ethylc ellu lose, the lower ends of the lead wires being connected by bridge ,wire 4. The bridge plug is also provided'with an annular steel ring 5 about its periphery. The-plug was made in accordance with standard moldingprocedures with'the steel ring forming a metal insert. -It will be noted that the outer face of the ring 5 isflush with the lateral surface of the bridge plug. v v v j Figure 2 represents a modified type of bridge plug utilized in carrying out this invention. In this embodiment, the steel ring has been replaced by two turns of 30 gauge iron wire 6 positioned within the peripheral groove 7 of the bridge plug.' Here again the external diameter of the coiled iron wire is substantially the same as the diameter of the bridge plug.

In Figure 3 the bridge plug of Figure l is shown positioned in the open end of an ethylcellulose detonator case 8 which may be loaded with any suitable explosive train shown generally at 9. The bridge plug 1 forms a slidingfit with the internal surface of the detonator case and the steel ring 5 is also in contact with the interior surface of the detonator case. The assembled detonator is positioned in the center of an induction heating gens erator work coil shown diagrammatically at 10. The

depending uponthe particular method by which the current is induced through the metal ring.

Detonators assembled in the manner shown in Figure 3 were sealed by placing them in the center of the work coil and maintaining the coil at an energy level of about 1 kilowatt for 2 seconds. The current thus induced in the steel ring heated it sufiiciently to melt, fuse, and

, intermingle the ethylcellulose in the plug and case about the ring so as to provide an effective seal between the plug and-the case. Figure 4 illustrates an assembled plasti c electric detonator that has been sealed in accordance with thisprocess. it will be noted that the ethylcellulose immediatelyabout the steel ring 5 has fused together as shown generally at 11 so as to merge the plug and case into a composite unit. I

Ten detonators sealed in this fashion were placed in a water bath and'subjected to a pressure of approximately 100 p.s.i for 24 hours. At the end or" that time, 'the detonators were removed from the water and inspected to determine whether any water had entered the case. All of the caps thus tested successfully passed this test withoutpermitting any water to enter the detonator case.

j Detonators in which coils of .30 gauge iron wire about the bridge plug were used instead of a steel ring were also sealed in accordance with the process described above. 'Detonators having ethylcellulose bridge plugs and cases and having 2 turns of :30 gauge iron wire placed in a peripheral groove about the bridge plug as shown in Figure 2 were placed in the workcoil and heated for periods ranging between .2 and 5 seconds by main-- taining the work coil at an energy level of about 1 kilowatt. Fifteen detonators sealed in this fashion failed to show any evidence of leaking when submerged in water under a pressure of 100 p.s.i.-for 24 hours.

In like manner, another series of detonators was prepared in which 3 turns of iron wire were coiled about .the peripheral .groove of the bridge plug. These detonators were sealed by maintaining them in the work coil .for 3 seconds with the coil at an energy level of 1 kilowatt. Each of these detonators also successfully passed the above described water immersion test.

The term electric detonator is used herein in its broadest sense and includes electric squibs, electric'blasting caps and other types of electrically actuated explosive igniters and initiators.

Since variations in the specific embodiments which have been described may be made within the spirit and scope of this invention, the detailed description is not to be considered as limitative except in the light of the appended claims,

Having thus described the invention, what is claimed 1. A method of sealing a substantially cylindrical electric detonator having a thermoplastic case and a substantially cylindrical thermoplastic bridge plug which comprises providing a thermoplastic bridge plug with a peripheral groove intermediate the ends of the plug, substantially filling said groove with an annular metallic member, inserting the plug and the metallic member into i the open end of the thermoplastic case containing an explosive charge with the annular metallic member contacting the thermoplastic case and being separated from the explosive charge by a, substantial portion of the bridge plug, and elevating the temperature of the metallic member above the melting point of the thermoplastic components of the detonator by an induced electric current whereby the thermoplastic material of the bridge plug and the thermoplastic material of the case adjacent the annular metallic member melts and intermingles to form an autogenous Weld between the bridge plug and the case.

2. A method ofsealing a substantially cylindrical electric detonator having a thermoplastic case and a thermoplastic bridge plug with a pair of lead wires passing therethrough which comprises providing a substantially cylindrical thermoplastic bridge plug having a metal ring molded in a groove about the periphery of the plug intermediate its ends, inserting the bridge plug intothe open end of a thermoplastic detonator' case containing an ex plosive charge and positioning the metal ring in contact with the thermoplastic case, the metal ring being separated from the explosive charge by a substantial portion of the plug, and elevatingrthe temperature of the ring current.

Whilethe above embodiments are concerned with det- Y onators provided with ethylcellulose components, other thermoplastic materials can be used.' Thus, polyvinyl chloride, polyvinyl acetate, polyvinyl chloride-acetate,

,polystyrene, polyethylene, methyl styrene, acrylonitrile,

. 0r,polymethy1 styrene, .and the .like can valso'be used in the formulation of :electric detonators in accordance with this invention. 1

7 References Cited in the file of this patent UNITED STATES PATENTS 7 2,722,155 Pauls Nov. 1, 1955 2,757,566 Towell et al Aug. 7, v1956 2,767,655 Seavey Oct. 23,1956 2,802,421 Home et al Aug. 13, 1957 2,802,422 Home Aug. 13, 1957 2,818,020 Burklund Dec. 31, 1957 FOREIGN PATENTS 652,054- Great Britain A r. 18, 1951 

1. A METHOD OF SEALING A SUBSTANTIALLY CYLINDRICAL ELECTRIC DETONATOR HAVING A THERMOPLASTIC CASE AND A SUBSTANTIALLY CYLINDRICAL THERMOPLASTIC BRIDGE PLUG WICH COMPRISES PROVIDING A THERMOPLASTIC BRIDGE PLUG WITH A PERIPHERAL GROOVE INTERMEDIATE THE ENDS OF THE PLUG, SUBSTANTIALLY FILLING SAID GROOVE WITH AN ANNULAR METALLIC MEMBER, INSERTING THE PLUG AND THE METALLIC MEMBER INTO THE OPEN END OF THE THERMOPLASTIC CASE CONTAINING AN EXPLOSIVE CHARGE WITH THE ANNULAR METALLIC MEMBER CONTACTING THE THERMOPLASTIC CASE AND BEING SEPARATED FROM THE EXPLOSIVE CHARGE BY A SUBSTANTIAL PORTION OF THE BRIDGE PLUG, AND ELEVATING THE TEMPERATURE OF THE METALLIC MEMBER ABOVE THE MELTING POINT OF THE THERMOPLASTIC COMPONENTS OF THE DETONATOR BY AN INDUCED ELECTRIC CURRENT WHEREBY THE THERMOPLASTIC MATERIAL OF THE BRIDGE PLUG AND THE THERMOPLASTIC MATERIAL OF THE CASE ADJACENT THE ANNULAR METALLIC MEMBER MELTS AND INTERMINGLES TO FORM AN AUTOGENOUS WELD BETWEEN THE BRIDGE PLUG AND THE CASE, 